A final goal of diabetic treatment is to prevent the onset of diabetic complications and to inhibit the development thereof. As demonstrated by clinical tests for achieving this goal it is important to find any abnormality and start treatment thereof at the earliest possible stage [e.g., Diabetes Research and Clinical Practice, 28, 103 (1995)].
Further, it is considered effective as a more advanced preventive method to find individuals with prediabetes or at prestage of diabetes, or individuals with mild impaired glucose tolerance or insulin secretory defect, who are not prediabetic at present but are highly likely to develop diabetes or prediabetes in the near future, and give them treatment or advice for exercise and dietary. Clinical tests have been conducted to scientifically demonstrate this [e.g., Diabetes Care, 21, 1720 (1998)]. Therefore, detecting individuals with prediabetes will be important for prevention of diabetes mellitus and also complications thereof. Furthermore, diagnosing individuals with mild impaired glucose tolerance or insulin secretory defect, who are not prediabetic at present but are highly likely to develop diabetes or prediabetes in the near future, is considered most important for purpose of preventing diabetes mellitus at an earlier date.
An example of the diagnostic method for diabetes mellitus is an oral glucose tolerance test. After a 75 gram oral glucose load, a group of individuals with the fasting blood glucose level being less than 110 mg/dl and the 2-hour postload blood glucose level being less than 140 mg/dl is defined as normal glucose tolerance (NGT). In addition, a group of individuals with the fasting blood glucose level being not less than 110 mg/dl but less than 126 mg/dl and the 2-hour postload blood glucose level being less than 140 mg/dl is defined as impaired fasting glycemia (IFG); and a group of individuals with the fasting blood glucose level being less than 126 mg/dl and the 2-hour postload blood glucose level being not less than 140 mg/dl but less than 200 mg/dl is defined as impaired glucose tolerance (IGT); and both groups IFG+IGT are defined as borderline type. A group of individuals with the fasting blood glucose level being not less than 126 mg/dl or the 2-hour postload blood glucose level being not less than 200 mg/dl is defined as diabetes mellitus.
The guideline of the Japan Diabetes Society teaches that among individuals defined as NGT on the basis of only the fasting blood glucose level and the 2-hour postload blood glucose level, those with the 1-hour postload blood glucose levels being 180 mg/dl or more are at higher risk to develop diabetes so that they should be handled as the borderline type.
The term “impaired glucose tolerance” or “glucose tolerance failure” refers to the condition of an increase in blood glucose level caused by insufficient uptake of blood glucose into peripheral tissues such as skeletal muscle, liver, and adipocyte after glucose is introduced into the blood through meals. In addition, the term “mild impaired glucose tolerance” refers to that the increment is slightly higher than that of healthy individuals.
Insulin is a hormone secreted from beta cells of pancreas and acts on skeletal muscle, liver and adipose tissue to lower the blood glucose level. The term “insulin secretory defect” refers to the condition of insufficient insulin secretion to uptake a sufficient amount of blood glucose into peripheral tissues such as skeletal muscle, liver, and adipocyte after glucose is introduced into the blood through meals or the like. Among the insulin secretory defect, the condition of insufficient insulin secretion to uptake the blood glucose into peripheral tissues just after glucose is introduced into the blood is referred to as “impaired early insulin secretion”. According to the guideline of the Japan Diabetes Society, the term “impaired early insulin secretion” refers to the condition in which the insulinogenic index I.I is less than 0.4. Insulinogenic index I.I is defined as ΔIRI (30-0)/ΔPG (30-0) wherein ΔIRI (30-0) means between the difference between the blood insulin levels at 30 min after glucose load and before glucose load; and ΔPG (30-0) means the difference between the blood glucose levels at 30 min after glucose load and before glucose load.
Assays of blood glucose levels and insulin levels for those diagnoses are invasive procedures that require blood drawing more than once within a short time, giving the subjects considerable pains. Therefore, there is a need for a simple assay with lower invasiveness, which can solve these disadvantages, preferably a noninvasive assay.
On the other hand, the quantitative determination of myo-inositol in a biological sample has been considered useful for the diagnosis of diabetes mellitus and the following reports have been provided.
(a) In diabetes mellitus, there was an increase in the urinary myo-inositol level [Larner J. et al., New Eng. J. Med., 323, 373-378 (1990)].
(b) No difference was found between NGT and the borderline type with respect to the urinary myo-inositol level [Susumu Suzuki, Diabetes Care, Vol. 17, No. 12 (1994) 1465-1468].
(c) The borderline type (IFG, IGT) and diabetes mellitus showed higher urinary myo-inositol level than that of NGT (JP 2001-190299 A).
The above reports (a) and (b) show the results obtained by determining the urinary myo-inositol levels with GC/MS. Nevertheless, the data are problematic in reproducibility and reliability because they varied among different examiners. On the other hand, in the report (c), the results are more precise and reliable than those obtained by GC/MS because they are obtained by determining the urinary myo-inositol level with a high-sensitive myo-inositol assay reagent using an enzyme. In this way, the detection of the group with prediabetes has become possible.
However, the myo-inositol assay reagent used in the report (c) has problems including: (i) an insufficient lower limit of detection because of a narrow measurement range and the need of diluting a sample for determination of various urinary myo-inositol levels; and (ii) insufficient avoidance of effects of coexisting substances in urine, particularly glucose. Therefore, the detection of mild impaired glucose tolerance and insulin secretory defect, which fall within NGT, has been impossible.
Furthermore, if a subject is judged to be NGT on the basis of only blood levels before a 75 g oral glucose load and at 2 hours after the glucose load, such judgment does not reflect the change in blood level from 0 to 2 hours. For example, even individuals (with mild impaired glucose tolerance and insulin secretory defect) who keep a higher blood glucose level from just after the glucose load, and thus are highly likely to develop diabetes or prediabetes in the near future are practically classified in NGT. As used herein, the term “mild impaired glucose tolerance” is classified in NGT, but refers to a slight decrease in glucose tolerance characterized by, when the loading test is carried out and blood samples are collected four times on fasting and at 30 minutes, 1 hour, and 2 hours after the load, (i) an oxyhyperglycemia, i.e., very high blood glucose levels (180 mg/dL or more) at 30 minutes and 1 hour after the load, (ii) a higher blood glucose level than that of healthy individuals at 2 hours after the load although the blood level is less than 140 mg/dL (e.g. not less than 120 mg/dL), (iii) high ΣPG (e.g. 530 mg/dL or more), i.e. the total of blood glucose levels just before 75g oral glucose load and at 30, 60, and 120 minutes after the glucose load Therefore, it can not be anticipated from the public disclosures (a)-(c) to identify the group of individuals who are highly likely to develop diabetes or prediabetes in the near future, for example, individuals with mild impaired glucose tolerance, by determining myo-inositol levels.
In this way, the conventional technology teaches no method of detecting mild impaired glucose tolerance and insulin secretory defect, which keep a higher blood glucose level from just after the glucose load, and thus are highly likely to develop to diabetes or prediabetes in the near future.